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Complement degradation product C3d in urine: marker of lupus nephritis
V S Negi1, A Aggarwal, R Dayal
1Department of Clinical Immunology, Sanjay Gandhi Postgraduate Institute of Medical Sciences, Lucknow, India.
Objective:
To examine whether serum and urine C3d, a degradation product of C3, correlate with renal and extrarenal lupus activity.
Methods:
Serum and urinary C3d levels were measured by ELISA in 15 healthy individuals and 24 patients with systemic lupus erythematosus (SLE) (8 with inactive disease, 7 with active but nonrenal disease, 9 with active lupus nephritis). Disease activity variables like serum C3, C4, and anti-dsDNA antibodies were also measured.
Results:
The median serum C3d levels were significantly higher (p < 0.01) in patients with active (26 arbitrary units/ml; AU/ml) and inactive SLE (27 AU/ml) compared to healthy controls (11.25 AU/ml); levels were comparable in patients with active renal and extrarenal SLE. On the other hand, urine C3d was elevated only in patients with active SLE; its level was highest in patients with active lupus nephritis (0.87 AU/ml) compared to patients with active extrarenal diseases (0.31 AU/ml; p < 0.05), to patients with inactive lupus nephritis (0.06 AU/ml; p < 0.001), or to levels in healthy individuals (0.06; p < 0.001). Urine C3d showed stronger correlation with disease activity score (SLE Disease Activity Index) than serum C3, C4, anti-dsDNA antibodies, and serum C3d.
Conclusion:
Urine C3d is a good index of active lupus, particularly lupus nephritis.